Raise your own - it is not as hard as you think. You will need some extra kit but there are techniques, such as the Miller method, which don't require any special skills. It is often said all beekeepers should learn to raise their own queens although in fairness most do it as a by product of swarm prevention, but real queen rearing takes the craft to a new level and even if you are unsuccessful you will learn by the experience and be a better beekeeper.
If you have plenty of Bees on more than 8 frames why not take a frame of brood and eggs frame of food and put it in a nuc box. Keep an eye on it and make sure they have plenty of stores and dont starve. Bees are better at raising their own queens than introduced ones.
No, depends on temperament and how many colonies you want or wish to offer for sale.
Of 15 cells from a grumpy queen - possibly, even likely, all of them would have gone!
why not take a frame of brood and eggs
Because the queen produced that way may be inferior to alternative methods where there are more than sufficient bees to nurse the brood and she will be produced in an 'emergency' situation, not as a supercedure impulse.
Home-bred queens like that may have unwanted temperament issues (and others) and a later choice/selection of offspring, to go forward as the gene pool, is a much better way to go. That is not so easy with just one or two colonies, if one also expects, or wants, a decent honey crop.